1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a magnetic disk apparatus for information recording and a method of manufacturing the same.
2. Description of the Prior Art
As a magnetic recording medium for performing high-density recording such as a hard magnetic disk, a metal magnetic thin-film medium formed by a coating method such as a plating method, a sputtering method, or a deposition method is used.
Recording of information in a hard magnetic disk or reproduction of recorded information therefrom is performed by a CSS (Contact Start/Stop) system. In this system, the disk is rotated at a predetermined speed to form a thin air layer between a magnetic head and the magnetic disk. When the rotation of the magnetic disk is to be started or stopped, the magnetic head and the magnetic disk are moved relative to each other in a contact-frictional state. In the CSS system, abrasion on the contact surfaces of both the magnetic head and the magnetic disk is advanced by a frictional force of contact sliding. In addition, if fine dust is present during recording or reproduction, the magnetic head may be brought into contact with the surface of the magnetic disk at high speed to cause a large frictional force between the magnetic head and the surface of the magnetic disk, thereby destroying the magnetic head and or the magnetic thin film. Especially in a metal magnetic thin film medium, since a friction coefficient of the metal magnetic thin film is large and the thickness of the film is small, the above problem conspicuously arises.
To solve the above problem, i.e., to improve a corrosion resistance of the magnetic recording medium and lubricating properties on its surface, Japanese Patent Laid-Open No. 61-208620 discloses a magnetic recording medium in which a carbon protective film is formed on a metal magnetic thin film and a liquid lubricating agent is coated on the protective film. Also, Japanese Patent Laid-Open No. 61-160834 discloses a magnetic recording medium in which an inorganic oxide film consisting of, e.g., SiO2 formed on a metal magnetic thin film directly or via a first protective film and a liquid lubricating agent such as a fluorine-containing oil, e.g., perfluoroalkylpolyether is coated thereon. In addition, Japanese Patent Laid-Open No. 61-208618 or 61-220120 discloses a method of increasing a bonding force between a lubricating organic film and an inorganic oxide protective film to improve adhesion properties.
However, in the prior art in which the liquid lubricating agent is coated on the carbon protective film, a bonding force between the lubricating agent and the protective film is weak. Therefore, when the CSS is repeatedly performed, the effect of the lubricating agent is gradually reduced to increase a frictional force between the magnetic head and the magnetic disk medium. In addition, since the carbon protective film is abraded, no satisfactory abrasion or damage resistance can be obtained.
In the prior art in which the inorganic oxide film is formed as the uppermost layer of the protective film of the magnetic disk medium and the lubricating agent having a polar group or a functional group is coated on the surface of the inorganic oxide film, the bonding force between the protective film and the lubricating agent is increased. However, it is difficult to coat a lipophilic lubricating agent on the entire surface of the hydrophilic protective film without forming fine pinholes. Therefore, coating of the lubricating agent is sometimes not performed on the entire surface of the protective film to expose the protective film not having a solid lubricating effect on the surface of the magnetic disk medium. In this case, when the CSS operation is repeatedly performed, film destruction is caused on a portion not coated with the lubricating agent.
The present invention has been made to solve the above conventional problems and has as its object to provide a magnetic recording medium having a surface which is not deteriorated even by frequent contact with a magnetic head upon driving or stopping of the magnetic recording medium, and a method of manufacturing the same.
It is another object of the present invention to provide a magnetic recording medium which does not cause a large frictional force between a magnetic head and the surface of the magnetic recording medium and therefore does not allow the magnetic head to destroy a magnetic thin film, and a method of manufacturing the same.
It is still another object of the present invention to provide a method of manufacturing a magnetic recording medium, which can convert silicon in a protective film into a silicon oxide by oxidation at a high speed within a short time period, thereby manufacturing a magnetic recording medium at a low cost.
The present invention is a magnetic recording medium in which a magnetic thin film is formed on a substrate directly or via an undercoating film, a protective film is formed on the magnetic thin film, and a lubricating organic film is formed on the protective film, wherein the protective film contains carbon and silicon, and silicon in at least an interface in contact with the organic film and in the vicinity of the interface is partially or entirely a silicon oxide. That is, the magnetic recording medium of the present invention contains, as components of the protective film in contact with the lubricating organic film, carbon and a silicon oxide, or carbon, silicon and a silicon oxide.
In order to maintain solid lubricating properties of the protective film and improve adhesion properties with respect to the lubricating organic film to be overcoated, the silicon oxide in the protective film may be uniformly present in the thickness direction of the protective film or may have a concentration distribution in the thickness direction. In particular, the silicon oxide is preferably contained in a large amount in an interface with respect to the lubricating organic film and in the vicinity of the interface and contained in only a small amount or not contained at all on the magnetic thin film side because the solid lubricating properties and the adhesion properties can be maintained and magnetization characteristics of the magnetic thin film are not degraded by oxidation upon formation of the protective film on the magnetic thin film.
The content of silicon in the protective film according to the present invention is preferably 1% to 30% (atomic %) with respect to the content of carbon. If this value is less than 1%, an Sixe2x80x94O coupling is hardly generated at the surface, so that an adhering force of oil is not improved and the effect is not entirely attained. In turn, in the event that a sufficient amount of oil is applied on the surface, an amount of wear at a continuous vibrating test (a drag test) at the head or a continuous sliding test (a pin disk test) at the same ball material as that of the head member is preferably less as an amount of silicon is increased. However, in turn, in the even that the value is more than 30%, the Sixe2x80x94C coupling in the film is increased, resulting in that a film breakage form is changed from a mild wearing as that of carbon less than that to a severe wearing (a wearing with the film breakage) and an anti-scaring characteristic is decreased and the head is easily crushed if the head is contacted with the disk during CSS. in addition, in the even that an amount of oil is less or that a partial loss in one disk is found, the film is broken into CSS and so this is not preferable. A value less than 30% indicating a mild wearing is preferable. A more preferable value is 5 to 25% . An effect of improving an adhering force of oil with Sixe2x80x94O coupling at the surface is remarkable at a value more than 5% and a CSS characteristic is improved, so that this value is more preferable. In addition, a value of amount of Si less than 25% is more superior in view of an anti-damage characteristic and this is more preferable even in the event that the oil is not applied (a degree of wear through the aforesaid continuous sliding test), it is more preferable. This value is more preferably 5 to 15%. An anti-wearing characteristic in the event that oil is not applied (a degree of wearing under the aforesaid continuous sliding test) is preferable if the less amount of Si is applied and in turn if the oil is applied, it is preferable that a large amount of Si is applied. In view of both characteristics, a maximum preferable amount of Si is 15%. In addition, the most preferable amount is 10 to 15%. A stable and preferable range of an actual production in the aforesaid 5 to 15% value is within 10 to 15%. That is, in view of the fact that the upper-most surface of the disk before applying oil is contaminated by some contaminations, the oil may not be sufficiently held under a value of 10% or less and the CSS characteristics is deteriorated. In this case, if a fine pinhole or a portion where adhesion strength is locally weak is present in the lubricating organic film coated on the protective film, the organic film may be removed from the protective film to cause the protective film to be brought into direct contact with a magnetic head upon sliding of the magnetic head. The damage resistance means a resistance with which a film damage which may disable recording/reproduction of information is not easily caused even in this state.
The thickness of the protective film is preferably 5 to 40 nm. If the thickness is less than 5 nm, the abrasion and damage resistance are significantly decreased. If the thickness exceeds 40 nm, a distance between a magnetic head and the magnetic thin film is increased too much to degrade the characteristics of the magnetic disk. When the layer containing a silicon oxide is to be formed in the vicinity of the interface with respect to the lubricating organic film, the thickness of the layer is preferably at least 3 nm. If the thickness is less than 3 nm, the adhesion strength with respect to the lubricating organic film is not increased. The silicon oxide in the protective film is so formed as not to degrade the magnetic characteristics of the magnetic thin film by oxidation.
In the present invention, a nonmagnetic metal thin film can be formed between the magnetic thin film and the protective film. Although the type of the nonmagnetic metal thin film is not particularly limited, a film consisting of Cr, Ti, Zr, Nb, Ta, W, and Mo or an alloy film consisting of two or more types thereof is preferably used as a barrier film for preventing oxygen in the protective film from being diffused into the magnetic thin film. In particular, a Ti or Cr metal film is preferably used because the film takes oxygen into the film to form a passive film and this passive film prevents diffusion of oxygen into the magnetic thin film. The thickness of the nonmagnetic metal thin film is preferably 1 to 10 nm. If the thickness is less than 1 nm, the film cannot satisfactorily function as a barrier film against diffusion of oxygen. If the thickness exceeds 10 nm, a distance between the magnetic head and the magnetic thin film is increased too much to degrade the recording/reproducing characteristics. The thickness is most preferably 1 to 5 nm to essentially prevent oxidation of the magnetic thin film and degradation in characteristics of the magnetic disk.
As the lubricating organic film used in the present invention, a film of perfluoropolyether is preferably used. Perfluoropolyether having an atom group such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94NCO or an aromatic ring at the terminal end of an organic molecular chain is more preferable because it is strongly bonded to a silicon oxide on the surface of the protective film. Perfluoroalkylpolyether having an aromatic ring at its terminal end and not having a polar group or a reaction group in its molecular is most preferable because it is strongly adhesive to a silicon oxide on the surface of the protective film and has a small friction coefficient which realizes good running characteristics of a magnetic head.
The second aspect of the present invention is a method of manufacturing a magnetic recording medium in which a magnetic thin film is formed on a substrate, an abrasion-resistant protective film is formed on the magnetic thin film, and a lubricating organic film is formed on the protective film, comprising the first step of forming the protective film consisting of carbon and silicon by performing sputtering using a target consisting of carbon and silicon as a cathode in a vacuum tank in which a reduced-pressure atmosphere can be formed, and the second step of heating and oxidizing a part or all of silicon present on at least a surface of the film formed in the first step in an atmosphere containing oxygen before formation of the organic film.
In the sputtering method performed in the reduced-pressure atmosphere in the first step of forming the protective film of the present invention, a method in which a target which consists of a sintered body obtained by mixing a fine carbon powder and a fine silicon powder and has electrical conductivity not causing charge-up of an electrical charge is sputtered by using a DC power source is preferably used. In the event that a sputtering device to be applied for a film formation is such a device having a degree of vacuum of 3xc3x9710xe2x88x927 Torr or less during the sputtering operation, a slight amount of oxygen gas or the like is added to argon gas and thus a film in which silicon in the film is slightly oxidized can be attained. In turn, also in the event that a degree of vacuum in the sputtering device is not full, a film similar to that attained under an addition of oxygen can be substantially attained. In case of the aforesaid two applications, it is possible to make a substantial reduction of the second stage of oxidization. An inert gas such as argon is used as the reduced-pressure atmosphere, and an oxidizing gas such as oxygen may be added in an amount not producing an oxide on the target surface to render sputtering unstable. The film coated in the first step may be a film consisting of carbon and silicon and not essentially containing oxygen or may contain a silicon oxide. In the above sputtering, although the substrate need not be heated, it may be heated within a temperature range not causing oxidation of the magnetic thin film to degrade the magnetic characteristics. A heating operation may be applied. In view of a anti-wearing characteristic of the film, as a temperature of the substrate during a film formation is higher, its amount of wearing is reduced, so that a temperature of the substrate less than about 300xc2x0 C. is more preferable. A cause why the anti-wear characteristic is improved is assumed to be based on the fact that the film is more condensed and a close fitting force of the film is more improved.
Oxidation of the protective film in the second step of the present invention can be performed by heating the film in the air or an atmosphere containing oxygen at a reduced pressure. By arbitrarily selecting an oxygen concentration in the atmosphere, a heating temperature, and a heating time, silicon concentration in the film containing carbon and silicon can be partially or essentially entirely oxidized into a silicon oxide in at least a portion near the surface of the film or throughout a thickness direction of the film. A temperature range of (200 to 400xc2x0 C. is preferable and further a temperature more than 250xc2x0 C. in which an oxidization process time is short is more preferable and further it does not influence over the substrate or the like and a temperature of less than 300xc2x0 C. is preferable in view of a cost of the oxidization processing facility. that is, a temperature of 250 to 300xc2x0 C. more preferable. An example of the magnetic thin film usable in the present invention is a metal magnetic thin film such as a cobalt-nickel-chromium alloy thin film.
As the method of forming the lubricating organic film on the protective film usable in the present invention, a method in which a lubricating agent such as well-known perfluoroalkylpolyether (e.g., Fomblin AM 2001 (tradename)) is dissolved at a proper concentration in an organic solvent and the resultant solution is coated and may be heated as needed can be used. In particular, perfluoroalkylpolyether having a reactive group or an aromatic group at the terminal end of a molecular chain is preferably because it has good adhesion properties with respect to the protective film.
The third aspect of the present invention is a method of manufacturing a magnetic recording medium in which a magnetic thin film is formed on a substrate, an abrasion-resistant protective film is formed on the magnetic thin film, and a lubricating organic film is formed on the protective film, comprising the first step of forming the protective film consisting of carbon and silicon by performing sputtering using a target consisting of carbon and silicon as a cathode in a vacuum tank in which a reduced-pressure atmosphere can be formed, and the second step of oxidizing, using an oxygen ion plasma, a part or all of silicon present on at least a surface of the film formed in the first step before formation of the organic film.
In the sputtering method performed in the reduced-pressure atmosphere in the first step of forming the protective film of the present invention, a method in which a target which consists of a sintered body obtained by mixing a fine carbon powder and a fine silicon powder and has electrical conductivity not causing charge-up of an electrical charge is sputtered by using a DC power source is preferably used. An inert gas such as argon is used as the reduced-pressure atmosphere, and an oxidizing gas such as oxygen may be added in an amount not producing an oxide on the target surface to render sputtering unstable. The film coated in the first step may be a film consisting of carbon and silicon and not essentially containing oxygen or may contain a silicon oxide. In the above sputtering, although the substrate need not be heated, it may be heated within a temperature range not causing oxidation of the magnetic thin film to degrade the magnetic characteristics. In view of an anti-wear characteristic of the film, as a temperature of the substrate during film formation is high, its amount of wear is less, resulting in that it is preferable to perform a heating of less than about 300xc2x0 C. A cause of improving an anti-wearing under a heating of the substrate is assumed to be a fine structure of the film and an improved close fitness of the film.
Oxidation of the protective film performed in the second step of the present invention is performed by exposing the surface of the film coated in the first step and mainly consisting of carbon and silicon to an oxygen plasma. This oxygen plasma can be generated by a well-known plasma generating apparatus for applying an RF voltage. In addition, as a method for performing an oxidization, there is a method for oxidizing from a film surface under pressure of ozone. This oxidization process with this ozone has no need of heating operation and its device is simplified. In addition, this process is superior due to the fact that its processing time can be shortened.
Examples of the substrate used in the present invention are a glass substrate, a ceramic substrate, and an aluminum substrate.
The protective film according to the present invention contains carbon and silicon, and silicon is partially or entirely oxidized into a silicon oxide in at least the interface of the film which is in contact with the lubricating organic film. The protective film has solid lubricating properties. The lubricating organic film coated on the protective film comes in contact with the carbon component of the protective film and is strongly bonded to the silicon oxide component of the protective film. That is, the silicon oxide component in the protective film of the present invention improves the adhesion properties with the lubricating organic film, and the carbon component in the protective film imparts the solid lubricating properties to the protective film. Therefore, even if the lubricating organic film coated on the protective film is locally peeled, the carbon component is exposed to the surface to maintain the solid lubricating properties. At this time, since a friction coefficient with respect to a magnetic head is not increased, a damage to the magnetic thin film caused by head crash can be prevented.
It is important that a carbon film containing Si can be formed into a film under a CD sputtering showing a high mass production. A formation with another film forming method utilizing high-frequency wave is substantially meaningless. That is, its device cost is high and a poor stability in case of production is found. In addition, a carbon film containing Si may also be manufactured by a CVD method using a high frequency wave. The film made by this method contains a substantial amount of hydrogen without fail, so that this film results in such a film as one having a different feature from a film made under a sputtering method. That is, in order to improve a saturating characteristic with fluorine oil, it is necessary to cause a non-coupled element in the film surface at least and in case of carbon containing Si, it is necessary that Sixe2x80x94Oxe2x80x94 should be present. However, in case of the film made under a CVD method, a hydrogen h is coupled with Si to make Sixe2x80x94H. Accordingly, the non-coupling element is eliminated, so that an affinity with the oil is deteriorated and an addition of Si in carbon is meaningless.
The above, and other, objects, features and advantages of the present invention, will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings in which the same or corresponding parts are identified by the same reference numerals in the several views.